Heat a pan and fry butter and shallots for 3 minutes until softened.
Add 125g potatoes and 450ml stock and simmer for 9 minutes until tender.
Add 225g watercress and simmer for 2 minutes before cooling slightly.
Blend the soup in a food processor until smooth.
Return soup to the pan and stir in 150ml cream, salt, pepper, and nutmeg.
Boil 400g potatoes in water for 10 minutes until tender and drain.
Add 75g butter and chives to the boiled potatoes and crush with a fork.
Whisk 110ml cream with lemon juice and seasoning to soft peaks.
Serve soup around a spoonful of potatoes topped with cream and watercress.